1. Field of Invention
The present invention relates to an engine stop control system for a hybrid electric vehicle (HEV). The vehicle has a power train comprised of an engine, a first clutch, a motor/generator, a second clutch, and driving wheels arranged in this order as a torque transmission path The vehicle has two driving modes by selectively controlling an engagement or release of the first and second clutches so that the vehicle may be either electrically driven by the motor/generator only, or may be driven by both the engine and the motor/generator under a hybrid mode.
2. Description of Related Art
In a conventional hybrid vehicle of this type, a so-called one-motor, two-clutch, parallel hybrid vehicle is known in which a motor/generator is interposed between the engine and the driving wheels, and the power transmission path between the engine and the motor/generator may be selectively established by a first clutch, while the power transmission path between the motor/generator and the driving wheels may be selectively established by the second clutch.
The HEV may select an EV mode by releasing the first clutch with the second clutch engaged so that the vehicle is driven by the motor/generator only. The HEV may alternatively select a HEV mode by engaging both the first and second clutches so that the vehicle runs under a collaboration of the engine and the motor/generator.
In this one-motor, two-clutch parallel HEV configuration, due to the engine being disconnected from driving wheels by releasing the first clutch under the EV mode, no dragging torque of the engine will be experienced due to the engine, which has stalled as a result of a fuel cut (i.e., stopping the fuel supply). In other words, power loss due to the engine friction would not occur so that the EV mileage may be extended by reducing power consumption during the period of EV mode.
For the same reason, during a deceleration period when an accelerator pedal of the vehicle is released, energy recovery will not be sacrificed due to the engine friction, and the motor/generator may recover additional energy corresponding to the engine friction with improved energy efficiency.
An engine stop control technique of the HEV of a one-motor-two-clutch type is known. According to such an engine stop control system, in response to an engine stop associated with a driver's release of an accelerator pedal during a vehicle running state, engine will be controlled to stall or stop and the first clutch released, while maintaining the second clutch being connected.
However, in this engine stop control strategy, the engine lowers its rotation speed by the friction of the engine itself to finally come to stop with the first clutch released. Consequently, it is impossible to stop the engine at a specified crank angle position, but rather the stop position is left to take its course of nature and thus unpredictable.
When the power from the motor/generator only becomes insufficient, such as in the situations of an accelerator pedal being depressed, or when a need arises to charge a battery at a lower charged or storage state, the ease with which the engine restarts, i.e., an engine start-up performance, is greatly dependent on the crank angle at which the engine has come to stop. Therefore, in existing systems for engine stopping in which the engine stop angle will be determined by course of nature, a desirable engine stop position is not assured and the engine start-up performance remains unstable.
An alternative system has proposed a technology to control the engine stop crank angle at the position, at which an engine restart may be made easier by making use of braking force of generator which is directly connected to the engine to improve engine start-up performance. However, when applying such an engine stop crank angle control technique to the HEV of one-motor, two-clutch type, the result would be such that, during an engine stopping control, the second clutch will be released while maintaining the first clutch connected so that the rotation speed of motor/generator will be kept the same as that of the engine until the engine comes to a stop. This entails that an engine stop crank angle control is performed with the second clutch released and thus the motor/generator being released from the driving wheels. As a result, during an engine stop crank angle control period, a regenerative braking control by the motor/generator will not be able to be conducted so that worsening problem of energy recovery might be encountered.